Nature has skillfully given each of us a nose, which includes, among other things, nose hair and a secretion gland to kill all bacteria entered, and the ability to change a cold air inhaled into a warm air for protecting our breathing system. On comparison, our mouth has no such functions. It is the God's will that we should breathe through nose, but not the mouth; the mouth should be closed completely, and should only exhale air, if necessary.
However, the conventional respirator usually has one piece to cover one's nose and mouth. An individual often opens his (or her) mouth in sleep when snoring; therefore, such an action would violate the principle of protecting the breathing system with nose, and may develop a cold or tonsillitis as a consequence.